Impure caprolactam, prepared by for example Beckmann rearrangement of cyclohexanone oxime, can be subjected to a number of purification steps to obtain caprolactam of the purity required for polymerisation to nylon 6. A possible purification step is the hydrogenation that can be carried out to hydrogenate unsaturated organic compounds that can be present in the impure caprolactam. The presence of these unsaturated compounds is disadvantageous because they can impair the physical-mechanical properties of the nylon-6 made by polymerizing caprolactam. The saturated organic compounds formed by hydrogenation do not adversely influence these physical-mechanical properties of the nylon-6 and moreover these compounds are more easily removed in for example a distillation following the hydrogenation.
Such a process is described in EP-A-138241. In the process as described in EP-A-138241 caprolactam is mixed with water, the so obtained aqueous caprolactam mixture is subsequently hydrogenated in the presence of a Raney nickel catalyst (example I) or a nickel on SIO2 hydrogenation catalyst (example II) suspended in the aqueous caprolactam mixture to be purified. The hydrogenation catalyst is subsequently filtered off and water is removed by distillation at atmospheric pressure. The remaining product is distilled at a pressure of 0, 8 kPa and a temperature of 123° C.
It has surprisingly been found that the caprolactam obtained in such process still has a high PAN number.
The object of the invention is therefore a process for the purification of caprolactam wherein the PAN number of the obtained caprolactam is further reduced.
The object of the invention is achieved in that the distillation column contains nickel in an amount sufficiently low such that ΔPANNi≦3, wherein
ΔPANNi=ΔPAN−ΔPANNi=0,
ΔPAN=increase of the PAN number of caprolactam during distilling,
ΔPANNi=0=increase of the PAN number of caprolactam during distilling under the same conditions but in a distillation column free of nickel.
Preferably, the amount of nickel in said distillation column is sufficiently low such that ΔPANNi≦2. More preferably, the amount of nickel in said distillation column is sufficiently low such that ΔPANNi≦1.
In a preferred embodiment, the amount of nickel in said distillation column is sufficiently low such that ΔPAN≦3. More preferably, the amount of nickel in said distillation column is sufficiently low such that ΔPAN≦2. Even more preferably, the amount of nickel in said distillation column is sufficiently low such that ΔPAN≦1.